Thermal cyclers are instruments commonly used in molecular biology for applications such as PCR and cycle sequencing, and a wide range of instruments are commercially available. A subset of these instruments, which include built-in capabilities for optical detection of the amplification of DNA, are referred to as “real-time” instruments. Although these can sometimes be used for different applications than non-real-time thermal cyclers, they operate under the same thermal and sample preparation parameters.
The core of a thermal cycler construct consists typically of: one or more thermoelectric modules (also: “thermoelements”), such as peltier elements, sandwiched in close thermal contact between the sample holder (also: “thermal block”) and heat sink elements, along with one or more sensors in each of the sample holder and the heat sink, thermal interface materials on either side of the thermoelectric elements to enhance close thermal contact, and mechanical elements to fasten all of these components together.
The important parameters that govern how well a thermal cycler operates are: uniformity, accuracy and repeatability of thermal control for ail the samples processed, ability to operate in the environment of choice, speed of operation, and sample throughput.
The uniformity, accuracy and repeatability of thermal control is critical, because the better the cycler is in these parameters, the more confidence can be placed in the results of the tests run. There is no threshold beyond which further improvement in these parameters is irrelevant. Further improvement is always beneficial.
The ability to operate in the environment of choice is less important for devices used in a laboratory setting where the samples are brought to it, but choices become limited when it is desired to use the instruments outside the laboratory and to bring it to where the samples are located. The two main concerns here involve the size and, thus, portability of the instrument, and the power requirements of the instrument. These two concerns are directly related, as the biggest, single component in most cyclers is the heat sink used to reject the waste heat generated by the cycling. If a thermal cycler were to be built such that it only required enough power to operate off an automobile battery, it would also use a smaller heat sink because less waste heat was being generated. By further ensuring that the heat sink is engineered to be of high efficiency, the size can be minimized further and the instrument would become portable enough to operate virtually anywhere on earth.
Thermal cycling speed is important not just because it is a major factor in determining sample throughput, but also because the ability to amplify some products cleanly and precisely is enhanced or even enabled by faster thermal ramp rates. This can be particularly true during the annealing step that occurs on each cycle of an amplification protocol. During that time, primers are bonded onto the templates present, but if the temperature is not at the ideal temperature for this, not non-specific bonding can occur which in turn can lead to noise in the results of the reaction. By increasing ramp rate, the time that the reaction spends at non-ideal temperatures is reduced. It should be noted that an increase in ramp rate can be achieved by reducing the thermal capacitance of the samples and sample holders being cycled, or by increasing the thermal power supplied to the sample holder. These two methods can both be used in combination to increase speed over what is possible from either one alone. It should also be noted though that any increase in power supplied places additional load on the heat sink.
In thermal cyclers using conventional heat sinks, the temperature variation of the heat sink where it touches the thermoelements is caused by highly mismatched heat flux zones on the input and exhaust sides of the base plate. Restated simply, the thermoelements are located in a small central area of the heat sink base plate (the heat flux input zone), while the heat sink fins cover a much larger area of the opposing side of the heat sink base plate (the heat flux exhaust zone). This mismatch results in more rapid and efficient flow of heat from the edges of the input zone than the center, and thus a hot spot naturally occurs on the heat sink surface at the center of the thermoelements. Consequently, strong spatial variations in passive heat transfer through the thermoelements take place, which reflects to the temperature distribution of the samples to be thermally cycled. The problem of this kind of prior art is illustrated in FIG. 1.
Problems related to efficiency and thermal uniformity of the samples have previously been addressed in several publications.
U.S. Pat. No. 6,657,169 discloses a solution, which takes advantage of additional heating elements attached to the sample holder in order to improve the thermal uniformity of the holder. However, besides increasing the uniformity, the heaters also increase energy consumption of the device and increase complexity of the system.
US 2004/0,241,048 discloses a device which has an additional thermal diffusivity plate made of highly conductive material attached to the heat sink in order to convey heat to the heat sink more uniformly.
U.S. Pat. No. 5,475,610 discloses sample holder and microtiter plate designs which are meant to provide improved thermal uniformity. MJ Research Catalog 2000 also discloses one device structure, in which attention is paid on the thermal university of the samples during heating and cooling.
U.S. Pat. No. 6,372,488 discloses a thermal cycler having several sets of heating and cooling elements arranged in an array. By controlling each of the elements individually, the heating or cooling of the sample block can be adjusted. However, this solution significantly increases the costs and amount of control electronics of the device.
The LightCycler 480 System by Roche includes a heat pipe inserted in the heat sink. This solution increases the costs and complexity of the heat sink and thus the thermal cycling devices having such a neat sink.
Using any of the above-mentioned methods of devices adds unwanted complexity to the final instrument in the form of added or parts which increase manufacturing costs and lower reliability. Using of additional active heating elements has the same disadvantages as noted above, but also power consumption is increased.